1. Field of the Invention
The present invention generally relates to lightning protection systems for aircraft and, more particularly, is concerned with lightning shields that can be conformed and bonded to the various compound geometric surfaces found on composite material aircraft.
2. Description of the Prior Art
Composite materials, such as carbon fiber/resin products, are increasingly being used in the aircraft industry to take advantage of their relatively high strength/weight ratio. It can be expected that increasing marketplace pressures for larger payloads and greater fuel economy will continue to encourage the use of composites. However, and not unexpectedly, these non-metallic materials also present several design problems that are new to the industry, among which is a vulnerability to lightning damage. The previous all metal skin provided an excellent conductive surface, which is now replaced, in whole or in part, with a resistive material. Composite aircraft exterior surfaces, or skins, perform poorly when exposed to lightning. Thus, composite surfaces must be shielded against the harmful effects of lightning strikes.
Lightning is a violent, discontinuous discharge of electrical current in the air, most often found inside or around cumulonimbus (thunderstorm) clouds. An aircraft flying in the vicinity of thunderstorms is susceptible to "participating" in the path of a lightning discharge. Recent experimental results ("Aircraft Jolts From Lightning Bolts", IEEE Spectrum, Jul., 1988, pp. 34-38) indicate, however, that aircraft actually initiate lightning more often than they intercept it. In the majority of aircraft strikes, as the airplane flies into a strong electric field, leaders of opposite charge form at the extremities of the aircraft such as at the nose, tail, and/or wing tips.
Air is a very poor conductor of electricity, and these leaders mark the nascent stage of a lightning strike, as the surrounding air begins to electrically "break down," forming the more conductive ionization channels. The leaders continue to develop bi-directionally and three-dimensionally around the airplane. This process continues at a rate determined by such factors as the ambient electric field strength and the particular characteristics of the electrical "circuit"--including the capacitances and conductivities of the aircraft and the leader channels. Given a sufficiently strong field, a lightning discharge will occur down a specific leader pathway or channel of ionized air. This lightning channel may persist for more than a second after the airplane surface has become a "part" of the channel, with this conductive pathway remaining relatively stationary while the airplane continues its forward motion. Such relative movement causes the forward attachment point to "sweep back" over the airplane's surface, (the so-called "swept-stroke" phenomenon) , resulting in an increased potential vulnerability for additional aircraft surfaces besides the leading edges and other curved surfaces that have a greater initial electric potential. New lightning attachment points can occur at any location on the aircraft surface, and adequate lightning protection requires a continuous protective shielding for composite materials over the entire aircraft surface.
Metallic aircraft encountering lightning will conduct the electric current of a strike across the skin of the aircraft, in most cases suffering little resultant damage. On the other hand, composite materials like graphite epoxy resins, are resistive conductors that inhibit current conductance. By way of comparison, a graphite composite will absorb nearly 2,000 times the energy absorbed by the same mass of aluminum. It has been shown that the intense current density of a lightning strike, frequently approaching 200,000 amperes in one second, with rates of current change observed to 380,000 amperes per microsecond, can vaporize or "puncture" the thin composite laminates that make up the skin of the aircraft. Once such penetration occurs further damage can be done as the lightning pathway "intrudes" on the avionics, power supply circuitry or other critical systems, and actual physical damage may result as this current surge runs amok inside of the aircraft. Electromagnetic energy may also enter the aircraft through other types of apertures. In addition to holes created suddenly by lightning attachment to nonconducting aircraft skins, other apertures include seams where skin panels meet and repaired locations on skin sections that have been previously damaged. Because present techniques for seam filling and body hole patching utilize a nonconductive body putty, an electromagnetic "aperture" remains after such body putty is applied. Other electromagnetic apertures include exposed conductors, such as antennas. Electromagnetic fields that enter the aircraft can wreak havoc with on board avionics. This problem is further aggravated by the increasing use of digital designs in modern avionics to control critical flight functions besides their traditional navigation and communication tasks. It is well known that digital circuits, as compared to analog circuits, have little tolerance for electrical disturbances. A chief goal of modern aircraft designers must be to ensure that electromagnetic fields are not permitted to breach the aircraft skin, where they may disrupt avionics, damage structural components, and perhaps injure passengers or crew.
In response to these design requirements, a number of different shielding structures have been proposed for use with the new composite materials. These solutions generally require embedding a metallic substance into the composite skin. After the composite surface has been so shielded, conductivity is improved and the high density lightning currents are harmlessly dissipated over the surface of the aircraft. Three principal techniques have been shown to reduce the hazards associated with lightning, and each has met with varying degrees of success, with each presenting its own unique drawback to the aircraft designer.
In the first technique, composite fibers and aluminum or copper wires are interwoven into a fabric-like mesh. These shielding meshes perform well but they add a great deal of excess weight to the aircraft, typically around 0.08 lb./sq. ft. (see Table 1). To place this in the context of a typical commercial airliner, such as a Boeing 767, having a surface area of approximately 15,000 sq. ft., this interwoven mesh will add 1,200 lbs. to the aircraft weight. The additional load resulting from using this type of shielding forces the aircraft designer to incorporate more powerful engines into his design. The larger engines carrying more weight will naturally burn more fuel.
A second technique is to employ a flame spray of metal, usually aluminum, which is applied to the exterior surfaces of the aircraft. Application thickness may vary between four and six mils. Although halving the weight of the first technique, the lack of uniformity in thickness of this technique may create thin spots on the aircraft skin that are prone to damage.
Aluminized fiberglass, the third technique, is a fabric composed of fiberglass yarns, the outside of which is coated with aluminum metal. The fabric is then bonded to the composite skin of an aircraft with adhesives. The interposition of fiberglass between the aluminum and the graphite epoxy prevents galvanic corrosion that would normally otherwise occur upon the joining of these two dissimilar materials. Nearly as heavy as the flame sprayed metallic coating, the thickness of the fabric poses a major drawback. This material is inflexible and is therefore difficult to apply to compound geometries such as fairings, struts, radomes, wing edges and like complex aerodynamic contours.
Consequently, a need exists for improvements in aircraft lightning shields that will be lighter in weight, of uniform thickness, and that will provide sufficient flexibility, such that the material can be easily applied to and around the compound geometries and apertures commonly found on the exterior surfaces of aircraft.